Control arrangements are used in a variety of applications, including but not limited to various elements of a building comfort system, or heating, ventilation, air conditioning (“HVAC”) system. One device within an HVAC system is an air handling unit.
An air handling unit is typically known as a device that blows heated or chilled air through the ventilation system of a building. A typical air handling unit includes a fan (or other air moving device) and heating and/or cooling coils. The coils are designed to receive a heating material such as steam or hot water, or cooling material such as chilled water or other coolant. The coils thus heat or chill the air as needed before it is blown through the ventilation system. In summer, the air handling unit may blow cool air through the ventilation system by receiving chilled water into a coil and then drawing the air to be blown over the coil. In winter, the air handling unit blows heated air through the ventilation system by receiving heated water or steam into a coil and then drawing the air to be blown over the coil.
As with many devices that operate within a building system, the air handling unit is a dynamic system. In any dynamic system, there is a transition time between its input and output signals. The transition time it takes for the output to reach about 63% of its steady-state value due to a step change in the input is called Time Constant. In a dynamic system where the output always needs some transition time to reach its steady state, there will always be a Time Constant. For example, if a heating system can respond to a step input to raise the temperature of a house by 3 degrees, then the time constant is the time it takes for the house to change by 3 times 63% or 1.89 degrees.
In addition to the time constant, other parameters that express the dynamics of a physical system are known as the static gain and delay time. The static gain, time constant, and delay time together provide information that allow a controller such as a PID or PI controller to be tuned. Tuning controllers based on the static gain, time constant, and delay time of the physical system to be controlled is known in the art.
Thus, when a control system is being initially configured or installed, it is advantageous to obtain the static gain, time constant, and delay time of that system so that the controller may be properly tuned. While some known advanced control schemes can adapt to the changes in operating conditions, they still often require some initial settings including at least a time constant for the system.
In order to obtain the time constant of a particular device, specific tests on the device, such as a “bump” test, are performed. One type of bump test providing a step value input is provided to a device or system, which eventually causes the system to change from an initial measurable output A to a final measurable output B. The time of the transition from output A to output B is measured, with the time constant being identified as the time it takes for the device or system output to achieve 63.1% of the change from output A to output B.
Such bump tests have often been used to determine the time constants associated with the supply air temperature control processes of an air handling unit. As is known in the art, a typical air handling unit may alter the temperature of the supply air in a number of ways, including using heating and/or cooling coils, or admitting more (cooler or warmer) outside air into the building. Each of those supply air temperature control processes has its own time constant. Prior art bump tests involve forcing the air handling unit to change the temperature using each of these processes, and determining the response time.
Bump tests have a disadvantage in that they are relatively time consuming, are labor intensive, and require technical expertise. The system is typically tested after installation. Thus, costly tests and expertise are needed on a job-site to program parameters into a control system that involves an air-handling unit.
There is a need, therefore, for a method or apparatus that reduces the amount of testing and expertise required to develop a time constant estimate for an air handling unit.